Conventionally, plasma-arc welding has been known. In this plasma-arc welding, a welding plasma torch is used. This welding plasma torch includes, for example, a rod-shaped electrode, a first nozzle which is provided to surround the electrode and which injects a plasma gas and a second nozzle which is provided to surround the first nozzle and which injects a shielding gas (refer to Patent Document 1).
According to this welding plasma torch, a voltage is applied between the electrode and a material to be welded to generate an electric arc while injecting the plasma gas from the first nozzle. As this occurs, the second nozzle injects the shielding gas so as to surround the electric arc to prevent nitrogen and oxygen in the atmosphere from flowing into a molten weld portion.
Incidentally, it is practiced to form a tailored blank material by butt welding two types of sheet materials having different thicknesses. In this case, when the welding plasma torch described above is used, a base material is dented along both edges of a weld bead to thereby form undercuts. In particular, in the sheet material which is thin, the thickness of the portion where the undercut is formed is largely reduced.
Consequently, there have been fears that a strength of the tailored blank material cannot be ensured.